Organic light-emitting diode (OLED) devices, also referred to as organic electroluminescent (EL) devices, have numerous well known advantages over other flat-panel display devices currently in the market place. Among these advantages are brightness of light emission, relatively wide viewing angle, and reduced electrical power consumption compared to, for example, liquid crystal displays (LCDs).
Applications of OLED devices include active matrix image displays, passive matrix image displays, and area lighting devices such as, for example, selective desktop lighting. Irrespective of the particular OLED device configuration tailored to these broad fields of applications, all OLEDs function on the same general principles. An organic electroluminescent (EL) medium structure is sandwiched between two electrodes. At least one of the electrodes is light transmissive. These electrodes are commonly referred to as an anode and a cathode in analogy to the terminals of a conventional diode. When an electrical potential is applied between the electrodes so that the anode is connected to the positive terminal of a voltage source and the cathode is connected to the negative terminal, the OLED is said to be forward biased. Positive charge carriers (holes) are injected from the anode into the EL medium structure, and negative charge carriers (electrons) are injected from the cathode. Recombination of holes and electrons within a zone of the EL medium structure results in emission of light from this zone that is, appropriately, called the light-emitting zone or interface. The emitted light is directed towards an observer, or towards an object to be illuminated, through the light transmissive electrode. If the light transmissive electrode is the lower or bottom electrode of the OLED device, the device is called a bottom-emitting OLED device. Conversely, if the light transmissive electrode is the upper or top electrode, the device is referred to as a top-emitting OLED device.
The organic EL medium structure can be formed of a stack of sublayers that can include small molecule layers and polymer layers. Such organic layers and sublayers are well known and understood by those skilled in the OLED art.
In an active matrix display device such as, for example, an active matrix OLED display or an active matrix liquid crystal display (LCD), the active matrix portion or component is first formed on a substrate. This active matrix portion is also referred to as a back plane of a device. Such back plane generally includes units of thin film elements which repeat in a two-dimensional pattern across a substrate surface.
Each repeating unit of thin film elements comprises a pixel of a display device, and can include thin film transistors dedicated to receive addressing signals (so-called switching transistors) and thin film transistors dedicated to actuate display elements which are disposed over the back plane (so-called current control transistors or drive transistors) and responsive to signals received from the switching transistors. The back plane can further include electrical wiring, X-direction driving circuits, Y-direction driving circuits, and a capacitor.
As is well known, each thin film transistor has a gate electrode, a source electrode, and a drain electrode. The drain electrode of each of the current control transistors or drive transistors needs to be accessible to provide an electrical connection between such drain electrode and a corresponding or associated display element which is to be formed over or above the back plane. Typically, the drain electrode of a drive transistor is electrically connected to an electrode layer of a display element such as, for example, a liquid crystal display element or an OLED display element to provide a display pixel.
Technology advances and manufacturing process improvements have resulted in ready availability of active matrix back planes which find applications in display devices of varied size, including relatively large flat-panel displays capable of providing full-color displays of video images. These advances and improvements have been largely driven by market forces in the arena of liquid crystal display panels.
Accordingly, particular designs of back plane electrical circuits and of drain-to-display electrode connectors have been optimized for such LCD applications and can include organic planarizing layers to planarize topological features of the back plane such as, for example, drain-to-display electrode connectors which can project upwardly from the back plane.
With particular reference to active matrix OLED display devices, organic planarizing layers have also been used to planarize topological features of the back plane prior to forming the display electrodes. However, it has been found that acrylic planarizing materials, used frequently to provide planarizing layers, have a propensity toward moisture penetration and moisture retention. Moisture, in the form of water molecules, can significantly reduce the operational lifetime of an OLED device. Accordingly, it is desirable to provide a back plane for an active matrix OLED device which does not require organic planarizing layers.